1. Field of the Invention
The present invention concerns a method of fabricating an aluminum surface capable of polishing the aluminum surface to a mirror-smooth state at sub-micron accuracy, which is utilized, for example, in the polishing of substrates for magnetic discs or the polishing of polygonal mirrors used in laser printers, etc.
2. Description of the Prior Art
The mirror-smooth fabrication methods for aluminum surfaces that have been put to practical use so far include electrolytic polishing, precise polishing using abrasive grains (i.e., lapping) and mirror finish cutting using a diamond tool (i.e., dia-turn).
Among these methods, the electrolytic polishing utilizes the electrolytic dissolving effect for anode metals (workpieces) and, since even microscopic protrusions can be dissolved, it is excellent in that an extremely smooth surface having mirror-like gloss can be obtained as compared with mechanical polishing. However, since relatively a large degree of unevenness (about 3-6 .mu.m Rmax) remains, this method involves a drawback of requiring a preceding levelling finishing step to some extent.
On the other hand, in the lapping or dia-turn, such additional levelling is not necessary and the accuracy at the finished surface is 0.2-0.3 .mu.m Rmax and 0.03 .mu.m Rmax degree respectively. Since less than 0.1 .mu.m Rmax of accuracy is necessary for the finished surface of magnetic discs of high memory capacity (10 MB/in.sup.2.) or the polygonal mirrors, the dia-turn method will become important in the fabrication of substrates for the magnetic discs, etc.
However, the dia-turn method has the following problems.
(I) Since the mechanical accuracy of a machine tool has a direct effect on the fabrication accuracy of a workpiece, it is necessary to maintain the mechanical accuracy and the required accuracy therefor is, for example, as high as less than 1 .mu.m.
(II) Since the setting accuracy of a diamond tool has a significant effect on the fabrication accuracy of a workpiece, considerable skill is required for the setting of the tool.
(III) Residual stresses generated in the workpiece is increased by the dia-turn cutting.
A method referred to as an electrolytic-abrasive polishing or electrolytic lapping has recently been developed as a method of applying mirror-smooth surface fabrication to stainless steels and ordinary steels. In this surface fabrication method, superfine fabrication is applied by combining the electrolytic anode dissolving effect and the polishing effect of removing protruded portions of passivative films formed at the surface to be fabricated by lapping with abrasive grains. Such a surface fabrication method can eliminate the drawbacks in the dia-turn method described above.
Referring to the electrolytic-abrasive polishing, conditions therefor have already been proposed for stainless steels and ordinary steels, capable of applying mirror-smooth fabrication as fine as the surface roughness of 0.01 .mu.m Rmax. However, conditions for the mirror-smooth fabrication of aluminum surface have not yet been proposed. The principal reason why such fabrication conditions have not yet been proposed for the aluminum material lie in the following points:
(I) Since the passivation region of aluminum is narrower as compared with that of stainless steels, the proper setting for the electrode voltage is difficult.
(II) Since the surface of aluminum material is softer as compared with stainless steels, the proper setting
(III) In addition, since pits tend to be formed due to the effects of inter-metallic compounds in aluminum, the conditions for the mirror-smooth polishing have not yet been proposed from this point of view.
In the composite electrolytic mirror-smooth fabrication, super finishing is applied to the surface to be fabricated by urging an abrasive head, which comprises an electrode secured to a rotary shaft and an abrasive cloth covering a buffer member secured to the lower end face of the electrode, under rotation to the surface to be fabricated under a predetermined urging pressure while supplying an electric current between the electrode and the surface to be fabricated. In order to attain the mirror-smooth surface, it is important to uniformly exert the predetermined urging pressure to the surface to be fabricated.
As the method capable of making the urging pressure uniform, there has been proposed a method of using an abrasive material in which abrasive grains are retained with viscoelastic material (refer to Japanese Patent Laid-Open no. Sho 58-137525) or a method of disposing a buffer member made of elastic material between an abrasive cloth and the electrode (refer to Japanese Patent Publication No. Sho 58-19412 and Japanese Patent Laid-Open No. Sho 56-139699). In this case, non-woven fabrics are usually used for the abrasive cloth and the buffer member since it is required, if free abrasive grains are used in the electrolyte, that the electrolyte and the abrasive grains should be permeated therethrough and in view of securing the working life of the abrasive cloth.
In the above-mentioned patent literatures, it has been described, for example, that a surface fabrication with the roughness of less than several tens of nm Rmax is possible in a case where the work is, for example, austenite stainless steels having a surface hardness as high as HV 200.
However, in the conventional composite electrolytic mirror-smooth fabrication, although a satisfactory mirror surface can be obtained for the workpiece having a high surface hardness, it is difficult to apply mirror-smooth fabrication to a surface roughness of less than 0.1 .mu.m Rmax in the case of soft material such as aluminum having a surface hardness as low as HV 70 since scratches are liable to be formed at the surface to be fabricated.